This invention relates to the removal and stabilization of chromate ions and other oxy-metal ions from liquid solution. Particularly, this invention relates to a method that makes use of barium compounds to remove chromate ions and other oxy-metal ions from liquid solutions.
For purposes of environmental protection, chromate ions are considered as very serious oxy-metal ions. Because of high toxicity, chromate ions can cause cancer or induce cell mutation in human as well as animal bodies. Chromate salts are widely used in electroplating, dye-works, chemical fertilizer production, petrochemical industry, tannery, textile industry, paper-mill, steel-works, and more. Therefore, research on the treatment and recycling of underground water, household waste water, or industrial liquid wastes containing chromate ions has received much attention all over the world. Presently, accepted methods for treating such liquid wastes involve methods such as: adsorption, reduction-precipitation, ion-exchange, solvent extraction, diaphragm electrolysis, inverse dialysis, etc.
U.S. Pat. No. 4,481,087 discloses an adsorption method, in which hydrous ferrous oxide (FeO(OH)) is used in processing to form a powdery, porous black material with high surface area, as adsorbent, whereby it adsorbs chromate ions or dichromate ions (Cr2O7xe2x88x922 and HCr2O7xe2x88x92). The method is far from satisfactory, in that one gram of the absorbent can at most absorb only 25 mg of hexavalent chromium ion Cr+6).
In U.S. Pat. Nos. 5,302,290 and No. 5,456,840, a complex salt agent, poly-di-methyl di-allyl ammonium chloride (i.e. PDMDAAC), is employed to react with the soluble Cr+6 in an aqueous solution, thus forming a chromic complex. This aqueous solution is then made to pass through an ultra-filter. The concentrated chromic complex mud gathered after the filtration process is mixed with barium chloride (BaCl2) or lead chloride (PbCl2) to form the almost insoluble barium chromate (BaCrO4) or lead chromate (PbCrO4) precipitates; meanwhile, complex cations react with chlorine ions, turning into the original complex salt agent which is perfectly suitable for reuse. By applying this reduction-precipitation method to underground water purification, chromate ions"" concentration can be reduced down to 42 ppm or below. However, because ultra-filtration is a time-consuming process, this method is of but very limited use.
U.S. Pat. No. 4,525,483 gives an example of ion-exchange resin technique, in which chlorine ion-exchange resin and hydrogen ion-exchange resin are mixed into a resin bed to remove CrO4xe2x88x922 from the spent coolant discharged from a water cooling system, or from the sodium chloride solution used in an electrolysis process for the making of sodium chlorate (NaClO3). The method is still not exactly satisfactory, in that 1 mg of chlorine ion-exchange resin can absorb only about 0.01 g of CrO4xe2x88x922.
A method for stabilizing chromate ions within cement is disclosed in U.S. Pat. No. 4,572,739, where hydrated ferrous sulfate salt, FeSO4.7H2O, or ferrous sulfide, FeS, is mixed into cementing materials in the process of cement production, so as to reduce the solubility of the soluble chromate salts within said mixture. However, in practice, it was quickly found that, thanks to the high moisture and high temperature conditions inside the cement pulverizer, chromate salts were partially dissolved, which trickled down and left traces on the conveying belts, causing skin irritation and other more serious health problems among the cement workers. So this chromate-ion stabilization method fails to attain its goal.
U.S. Pat. No. 5,211,853 discloses a reduction-precipitation method using hydroxyl amine, NH3.OH, or hydroxyl amine sulfate, (NH2OH)2.H2SO4 (i.e. HAS), for reduction agent. At controlled pH and temperature, said reduction agent reacts with potassium dichromate (K2Cr2O7) that has been used in the production of sodium chlorate, resulting in divalent and/or trivalent chromic hydroxide precipitates. After filtering out such chromate salts, residual Cr+6 concentration in the filtrate is found to be between 2 and 10 ppm.
U.S. Pat. No. 5,326,439 makes use of Andco electrolytic method to remove such soluble toxic elements as chromium (Cr), arsenic (As) and lead (Pb) from underground water. According to this disclosure, ferrous ions are formed at the iron-alloy anodes under suitable conditions, while in the electrolyte Cr+6 is reduced to Cr+3, and at the same time the barely soluble chromium trioxide (CrO3) precipitates are coming out. Thereafter, the precipitates are separated from the water by means of filtration. This is doubtless a very effective method for small-scale water purification, as residual Cr+6 concentration of such treated water is lower than 0.05 ppm.
Canadian Patents No. 1, 119, 772 and No. 1, 139, 080 disclose a reduction-precipitation method to reduce Cr+6 to Cr+2 and C+3, where hydroxylamine (NH3OH) or hydrazine (NH2.NH2) is the reduction agent which reacts with dichromate salts present in a chlorate solution, resulting in a blue-green hydrated chromium oxide precipitate (Cr3O4.xH2O). A distinct drawback of this method is that such reduction agents are as toxic as they are expensive.
Still one more reduction-precipitation method is found in U.S. Pat. No. 3,616,344, where sodium sulphite Na2SO3), or ammonium sulphite ((NH4)2SO3), or tin diethyl-oxide (Sn(C2H5)2O), being the Cr+6reduction agent of choice, is added into a chlorate solution, causing the precipitation of a trivalent chromium hydroxide. The precipitates are then separated from the solution by means of filtration or centrifugation. Furthermore, the patent discloses that the addition of soluble metal salts (e.g. zinc, cobalt, lead and copper salts) into chromate ions"" solution can cause the formation of insoluble chromate salts (e.g. PbCrO4) which are subsequently filtered out, resulting in the removal of chromate ions.
A similar reduction-precipitation method for removing chromate salts from sodium chlorate solution is disclosed in U.S. Pat. No. 4,086,150, where soluble sulphides and then ferrous salts are added into the solution to convert chromate ions into trivalent chromium compounds which afterwards are filtered out.
In Taiwan Patent Application No. 6211803 a unique approach to chromate ions"" removal and recycling is made public. Here a pipe is filled up with porous leech stones which are spread over with almost insoluble lead compounds such as lead hydroxide (Pb(OH)2) and lead carbonate (PbCO3). Then the two ends of the pipe are sealed off with cotton fibers. By putting through the pipe a waste water known to be containing chromate ions, chromate ions react with aforesaid lead compounds, resulting in insoluble lead chromate (PbCr)4). Since by this method some lead ions are present in the treated water, the water is conducted to pass through a cation-exchange resin to absorb and detain the lead ions. And finally, the lead chromate is retrieved from the pipe for use in painting.
Taiwan Patent Application No. 77106237 discloses a method to remove Cr+6 from a chlorate-salt aqueous solution which contains chlorite ions and dichromate ions. Said chlorate salts solution is obtained from electrolytic salt water. At pH 10.0 and temperature 80xc2x0 C., chlorine water and alkaline buffers are added into the solution, converting dichromate ions into divalent or trivalent chromium compounds which gradually become precipitates of a blue-green color. Thereafter, said precipitates are removed from the solution by means of filtration.
Taiwan Patent Application No. 78102127 discloses a solvent-extraction method for removing heavy metal ions anions or cations of chromium, nickel, lead and copper, from waste water, where organic phosphinic acid, or 2,2-diethyl hexethyl phosphoric acid (D2EBPA), or one of aliphatic ammonium group is employed as extraction agent to react with heavy metal ions, resulting in precipitates of complex compounds which are subsequently separated from the purified water. Then, according to the chemical characteristics of those heavy-metal complex compounds, a suitable quantity of ore acid or ammonium hydroxide (NH4OH) is added to reclaim the extraction agent.
Taiwan Patent Application No. 84112122 discloses a dry distillation method for the recycling of chromate salts from an electrolytic mud. Without adding water to the mud nor increasing its temperature, suitable measure of acidity agent or strong oxidization agent is mixed into the mud, thus either driving out trivalent chromium ions from the mud or converting them into Cr+6. These chromium ions are then separated from the mud in the form of chromate salts, which are useful in several industries.
In Taiwan, some studies relevant to the present invention show that some attempts have been made to use moss peat, fly ash, activated carbon, carbonized paddy, delayed leaf veins and hematite as adsorbent materials. But for their low adsorption capacities, these instances are interesting because they can reduce cost in processing or recycling. In such cases the adsorption mechanism relies on the fact that various particles of oxides of iron, aluminum, and silicon, etc., being the basic components of red iron ore, fly ash, or moss peat, once present in aqueous solution become positively or negatively charged on their surfaces, thus some repel and others attract chromate ions, and, as a result, chromate ions are absorbed by some of these oxides. Moreover, the inventors have tried activated carbon, carbonized husks and humus, and have found them all capable of absorbing chromate ions. One gram of activated carbon can absorb Cr+6 up to 123 mg or more; but then activated carbon is rather expensive.
The foregoing general survey of the available chromate ions"" removal or stabilization methods indicates that each method has one or more of the following drawbacks, namely: high energy cost, high material cost, dangerous chemicals, expensive equipment, complicate procedures, long operation time, low absorption capacity, difficult to recycle, etc. At present, only ion-exchange resin method is widely used in handling industrial liquid waste containing chromate ions.
In view of the drawbacks of the prior arts as above briefly described, the inventors herein disclose an effective adsorption method for removing as well as stabilizing chromate ions and/or other oxy-metals ions. The inventors have discovered that low-acidity barium salts are potent absorbents with respect to chromate ions and other oxy-metal ions, and their adsorption efficiencies are greater than all the adsorption agents mentioned in the foregoing survey. For example, one gram of barium silicate as an adsorption agent adsorbs more than 200 mg of saturated hexavalent chromium ions, far greater than all the known adsorbents for such case. Moreover, it may be well to note that the adsorption agent of the invention with respect to the adsorption of chromate ions is an anion-exchange reaction. When barium silicate undergoes an adsorption process, it represents an exchange reaction of silicate ions with chromate ions as follows:
CrO42xe2x88x92+BaSiO2(OH)2=BaCrO4+SiO2(OH)22xe2x88x92xe2x80x83xe2x80x83(1)
The adsorption reactions with barium carbonate and barium fluosilicate are respectively as follows:
CrO42xe2x88x92+BaCO3=BaCrO4+CO32xe2x88x92xe2x80x83xe2x80x83(2)
CrO42xe2x88x92+BaSiF6=BaCrO4+SiF62xe2x88x92xe2x80x83xe2x80x83(3)
All three produce barium chromate (BaCrO4) whose solubility is extremely low; consequently, with this kind of absorption treatment the residual concentration of Cr+6 within the final solution is also extremely lowxe2x80x94usually lower than 0.01 ppm with barium silicate as absorbent, for instance. And it is also shown in the above formula that in the aqueous solution chromate ions are replaced by silicate ions, carbonate ions, or silicon hexaflourite ions. When the original chromate compound is sodium chromate (Na2CrO4), the above three absorption reactions can produce sodium silicate (Na2SiO2(OH)2), sodium carbonate (Na2CO3) and sodium silicon hexafluorite (Na2SiF6) respectively, all of which are of higher alkalinity than sodium chromate which they replace, hence, after such a reaction, the solution""s pH value will be higher than before.
Experimental results reveal that a solution""s pH can greatly affect absorption efficiency. The inventors have discovered that, barium silicate, as a potent Cr+6 absorbent, can maintain its high absorption efficiency fairly unchanged in the solutions of pH values ranging from 6 to 10. The invention, therefore, is applicable to a wide range of aqueous solutions containing chromate ions and/or other oxy-metal ions; even in the case that the absorption reaction increases the solution""s pH, the invention is usually still very effective.
High adsorption efficiency is probably the most notable merit of the present invention. Another thing worthy of note is that the above chemical formulas are reversible reactions; that is, in one direction (to the right) they are chromate ions"" adsorption reactions, whereas in the opposite direction (to the left) they are desorptions of chromate ions from solid-phase absorbents back into the solutions. So another advantage of the invention is that the absorbents having been used can be regenerated for reuse, and this significantly lower the cost of liquid waste treatment. Moreover, after such a desorption process, the de-sorbed chromate ions can be gathered for use. The inventors have discovered that 0.5N sodium carbonate is quite an effective chromate-ion de-sorption solution; the higher the solution""s concentration the better the desorption result. Desorption efficiency, of course, has to do with the pH value of the solution. It is found that, when the desorption solution""s pH value is smaller than 8, sodium carbonate solution""s desorption efficiency is less than remarkable; however, from pH 9 to pH 10 desorption efficiency increases quickly, reaching its optimum efficience once above pH 10.
Additionally, still referring to the removal of chromate ions, the invention uses high-water solubility barium compounds such as barium chloride, barium hydroxide, etc., to prepare a water solution, which is then mixed with a liquid waste containing chromate ions and/or other oxy-metal ions. This induces the precipitation of insoluble barium-salts, and subsequently the precipitates are filtered out.
Still another application of the invention is to add water soluble barium compound powders or their aqueous solutions into solidification agents containing wastes of chromate ions or containing wastes of other oxy-metal ions, so that insolubility barium compounds are similarly formed by chromate ion and other oxy-metal ions thereby achieving stabilization of chromate ion and/or other oxy-metal ions. This stabilization method can notably decrease chromate and/or other oxy-metal ions"" TCLP dissolving rates, and increase the quality of waste solidified bodies containing chromate ions or containing other oxy-metal ions and the waste loading capacity of solidified bodies.
Besides chromate ions, the method of this invention, as detailed in the foregoing, is effective with respect to other oxyetal ions including oxy-molybdenum, oxy-wolfram, and oxy-vanadium ions, etc.
In the following, some laboratory examples are presented to further illustrate the method and advantages of the present disclosure. These embodiments of the invention, including their sample compositions, experimental procedures, operation conditions, testing methods and results, etc., only partly indicate the application range of the invention: they do not represent the full scope of the same and can not be regarded as limits to the same.